Project Summary: Microglia, in addition to their role as immune cells of the central nervous system (CNS), are critical players in the development and function of the CNS. Microglia are dynamic cells that interact extensively with neurons, allowing them to regulate neurogenesis, refine neuronal networks, and influence synaptic plasticity. The same properties that allow microglia to shape the development of the CNS may also lead to neurological disease if not properly regulated. The extracellular signals that mediate microglial dynamics and interactions with neurons are not well understood. Signaling through extracellular nucleotides has emerged as a key mechanism by which microglia sense and interact with their external environment. Specifically, the microglial P2Y12 receptor is crucial for microglial responsiveness to extracellular adenosine tri-phosphate (ATP) and mediates numerous microglial functions, including ATP-dependent chemotaxis, microglia-neuron interactions, and experience-dependent synaptic plasticity. However, little is known about the downstream signaling effectors that mediate these diverse actions of P2Y12. My preliminary data and previously published work suggest that P2Y12 activates phosphoinositide-3-kinase gamma (PI3K?), which in turn can activate the signaling pathways required for ATP- mediated microglial chemotaxis. In this role, PI3K? could translate localized extracellular ATP signals into directed microglial action and serve as a broad effector of P2Y12-dependent functions. In this proposal, I will test the hypothesis that microglial PI3K? mediates microglial ATP-dependent chemotaxis, microglial dynamics, and experience-dependent synaptic plasticity. In order to understand how PI3K? activation affects P2Y12-driven microglial behavior, I will first determine the role of PI3K? in directed microglial motility towards ATP, using in vitro, ex vivo, and in vivo approaches, and further dissect this molecular pathway, using a combination of pharmacological and genetic manipulations (Aim 1). In parallel, I will determine how genetic loss of microglial PI3K? affects microglial morphology, dynamics, and ocular dominance plastic in vivo, a process in which microglia are known to participate. These two lines of investigation will examine the molecular pathways underling distinct P2Y12-dependent microglia processes, and for the first time characterize the role of PI3K? in the unperturbed CNS. Uncovering these molecular mechanisms will be critical to understanding the role of microglia as mediators of neuronal function and their role in disease-related processes.